1. Field of the Invention
The present invention generally relates to fin control systems for gun-launched projectiles and, more particularly, to the actuators and associated linkages for controlling the angle of the aerodynamic control fins which impart steering forces to the projectile during flight.
2. Description of the Related Art
Presently existing actuators for fin control on gun-launched projectiles are both complex and expensive. The requirement to withstand the acceleration forces, which typically range from 10,000 to 30,000 G""s, places very stringent demands on the actuators. Therefore, the designs are required to be extremely robust in order to withstand the loads induced by these accelerations.
It is an object of this invention to provide a simple but effective configuration which is simple in structure but which is also low cost to produce and extremely robust. It is particularly capable of withstanding extreme accelerations.
At present, existing actuators for fin control on gun-launched projectiles typically employ electric motors to drive the fins through a gear reduction system. These motors are either brush or brushless types that make several revolutions of the motor while moving the fin from one travel limit to the other. In the case of the brush type motors, there are substantial reliability issues with the brush systems due to the high acceleration loads and problems with corrosion resulting from long-term storage. The brushless types have reliability issues with rotor position sensing complexity.
There are typically two basic configurations of gun hardened projectile actuators: (1) the motor on the central axis, and (2) the motor off the central axis to allow the fins to be folded adjacent to the motor. The design of the present invention addresses both configurations. This design is applicable to projectiles in which the fins are stowed internally and extended to effect steering control after the projectile is fired. The design is also applicable in those configurations in which the fins are not folded because they are small or short enough to fit in the gun barrel. Also, it is appropriate for projectiles where a sabot is used to adapt a projectile with external fins to the dimensions of the gun barrel, wherein the sabot opens up and peels away after the projectile leaves the muzzle.
Systems of the present invention comprise as the motive element a limited rotation motor, the shaft of which rotates through less than half a revolution in driving the fin from one travel limit to another. These limited rotation motors lend themselves to low cost manufacturing techniques. There is no requirement for secondary electronics to sense the motor commutation steps. Cost is reduced by not having to commutate the motor over its rotation. Prior art systems utilizing brushless motors operating over several revolutions require Hall effect sensors or some other means of sensing rotor position to switch phases. A brush-type motor would require brushes and a commutator. Either approach introduces added cost over what is required in the design of the present invention.
Limited rotation motors of the type used herein may be either a moving magnet type or a variable reluctance type wherein the rotor reluctance is used to torque the motor in the external armature field. Because the variable reluctance approach does not use permanent magnets, it has a potential for lower cost. Either of these types of motors is relatively cheap to manufacture. Either can be machine wound automatically so very little handwork is required. Also, each design is very robust and can be hardened for gun launching. In either case the inherent simplicity of the design makes for high reliability. This design also provides benefits in the case where a more conventional motor of either brush or brushless type is used. This is because, among other reasons, as the loads increase with increase of fin angle, the reduction ratio between the motor and the output shaft increases. This reduces the power needed to provide the required torque at the output shaft.
The relatively low reduction ratio between the motor and the fin shaft of the instant design is well suited to rapidly rolling airframes, for example up to a 30 Hz roll rate. This is because rolling airframes require fin deflections that follow the projectile roll without significant phase error. This allows maneuvering in a single plane without inducing spiraling. Thus, it is a feature of the present invention design that it is well suited to applications with rolling airframes.
Furthermore, there is potential for energy conservation in these rolling airframe applications because the aerodynamic induced torque applied to the control fin can be used to decelerate and re-accelerate the motor, thus recovering some of the electrical energy that was input to the motor. The degree and effectiveness of this energy recovery depends on several factors, among which are the motor and mechanical drive characteristics, the electronic driver, and the battery or other power source.
The change in the reduction ratio between the motor and the fin shaft as the angle of the fin increases comes about because the ratio from motor to fin shaft varies as the sine of motor shaft position divided by the sine of the fin shaft position. Since the motor shaft angular displacement is a multiple of the displacement of the fin shaft, the effective ratio increases as a function of displacement. Thus, the maximum ratio occurs at the extremes of travel where the aerodynamic load is greatest. This feature permits minimization of the motor size because of the inherent torque increase for maximum load.
Comparison of the cost of the embodiments of the present invention with known prior art arrangements is extremely favorable for the present invention. The simplest way to look at cost is to compare part count and complexity. The simple ball and crank pin and eccentric slotted coupling for the fin shaft are much lower cost than any gear or screw drives. These features are also much lower cost than precision bearing bores for gears.
In brief, particular arrangements in accordance with the present invention comprise a brushless motor having an output shaft which is controlled to rotate less than 90xc2x0 from either side of a rest position. The end of the motor shaft has a crank arm that is offset by a prescribed distance with a ball end machined on the end of the crank. In an alternative arrangement, a ball is mounted to the motor shaft by an eccentric pin. As the motor rotates, the ball slides in a slot in the fin shaft which is perpendicular to the motor axis. The ball and slot are at a radius of about one-half the prescribed offset distance, resulting in a motor-to-fin ratio of about 6:1. This sliding ball-and-slot coupling of the motor shaft to the fin shaft is a major feature of the present invention, since it takes advantage of the limited rotation of the fin shaft to eliminate the need for any gears or screw drives. Contact is maintained with the ball in the shaft slot as the shaft moves through its arc by allowing the ball to slide axially by a limited amount in the slot. The ball-to-slot interface is a tight slip fit. This arrangement results in a very high overall efficiency of the drive coupling, on the order of 90%.
In one particular arrangement of the present invention, the motor is mounted along the center line of the projectile, and engages the fin shaft through a right-angle mounted eccentric coupler.
In an alternative embodiment of the present invention, the motor is displaced from the projectile axis. The motor shaft is coupled to the output shaft by a link arm which permits the motor to be folded back into the projectile interior space, which also includes the battery envelope, thereby allowing use of the same motor and electronics of the other embodiment while further reducing total package length. In all cases, the design of the present invention allows for a very compact actuator drive package.